appending data to netCDF file

Hello:
I want to be able to append a netCDF file with data on different
levels, but cannot find this in the documentation. Let's say I have winds
at the surface and at 850 mb but defined as different variables. I just
want to stack the 850 mb winds onto the 1000 mb winds.
I tried:
save/file=wind.nc/k=1 wind
save/append/file=wind.nc/k=2 wind
etc....
so when i open up the file it should look like this:
wind i=1:x j=1:x k=1:2
where k=1 is the surface and k=2 is 850 mb.
Thanks for your help
Steve Guimond
Research Assistant
The Center for Ocean-Atmospheric Prediction Studies